Push-on-push-off electronic switch

Transistors Ql and Q2 make up the flip-flop while Q3 drives a reed relay. When power is first applied to the circuit, Ql and Q3 are conducting and Q2 is cut off. Momentarily closing SI causes the flip-flop to switch statesâ€”Ql cuts off and Q2 conducts. When Q2 is conducting, its collector drops to around 0.6 volt. That prevents base current from flowing into Q3 so it is cut off, de-energizing relay Kl. The flip-flop changes state every time SI is pressed. Capacitors Cl and C2 ensure that Ql is always the transistor that turns on when power is first applied to the circuit. When power is first applied to the basic flip-flop, the initial status is randomâ€”Ql and Q2 both try to conduct and, usually, the transistor with the higher gain will take control, reaching full conduction and cutting off the other one. However, differences in the values of the collector and coupling resistors will also influence the initial state at power-on.

With C2 in the circuit, it and R4 form an R-C network that slightly delays the rise in Q2's base voltage. That gives Ql sufficient time to reach saturation and thus take control.